1. Field of the Invention
The present invention relates to an electric power steering control apparatus for a vehicle such as a motor vehicle which is able to improve the convergence and control stability of a steering wheel during returning motion thereof with no steering load or torque being applied to the steering wheel by a driver or operator of the vehicle (i.e., hand free operation), as well as to reduce a burden to the driver when an excessive returning torque is applied to the steering wheel.
2. Description of the Related Art
FIG. 34 is a schematic view showing a conventional electric power steering control apparatus disclosed, for example, in Japanese Patent Laid-Open No. HEI 4-19270, and FIG. 35 is a circuit diagram showing a control unit of the apparatus. In FIG. 34, a steering wheel 201 is connected to a steering column 202 having a sensor 203 mounted thereon. The sensor 203 is connected to an electric power steering control circuit 204 which generates an output signal based on the signal from the sensor 203 for driving an electric motor 1. When the electric motor 1 is energized to generate driving power, an output shaft 205 of the motor 1 is driven to rotate the steering shaft 202 so that a pinion 206 attached to the steering column 202 is thereby rotated to movie a rack 207 to the right or left, i.e., in the lateral directions. The movement of the rack 207 causes front wheels 208 of the vehicle to be steered or turned to the right or left.
In FIG. 35, first to fourth field effect transistors (FETs) 21 to 24 cooperate with the electric motor 1 to constitute a bridge circuit and are connected to a logic control unit 25. The logic control unit 25 is connected to a central processing unit (CPU) 26. The logic control unit 25 includes a pair of AND circuits 25a, 25b and amplifiers 25c to 25g. The amplifiers 25c, 25e, 25f and 25g are connected to the first, the second, the fourth and the third FETs 21, 22, 24 and 23, respectively. The CPU 26 outputs a PWM signal S1, a forward rotation signal S2 to rotate the electric motor 1 in the forward direction and a reverse rotation signal S3 to rotate the electric motor 1 in the reverse direction. The first AND circuit 25a turns on the first FET 21 when the PWM signal S1 and the forward rotation signal S2 are input thereto from the CPU 26 at the same time. The second AND circuit 25b turns on the second FET 22 when the PWM signal S1 and the reverse rotation signal S3 are input thereto from the CPU 26 at the same time.
Also, if the forward rotation signal S2 is output from the CPU 26, the fourth FET 24 will normally be turned on. Likewise, if the reverse rotation signal S3 is output from the CPU 26, the third FET 23 will normally be turned on. Further a first current detector 27 for detecting a current during the forward rotation of the electric motor 1, a second current detector 28 for detecting a current during the reverse rotation of the motor 1, an amplifier 29 connected to a torque sensor 30, and a waveform shaping circuit 31 connected to a speed sensor 32 are connected to the CPU 26.
Next the operation of the above-mentioned conventional power steering control apparatus will be described. If the steering wheel 201 is operated by the driver, the torque sensor 30 and the speed sensor 32 sense a steering torque and a vehicle speed, respectively, and generate a corresponding torque signal and a corresponding speed signal at that time which are input to the CPU 26. Upon receipt of these signals, the CPU 26 calculates a command value for rotating the motor 1 based on the respective signals, and at the same time, the feedback current of the bridge circuit detected by the first current detector 27 is input to the CPU 26. Then, the CPU 26 calculates a PWM value based on a difference between the command value and the feedback current value and generates a corresponding output signal. Here, it is to be noted that the torque sensor 30 outputs a plus or positive signal when the steering wheel 201 is rotated in the forward direction and a minus or negative signal when the steering wheel 201 is rotated in the reverse direction.
If the steering wheel 201 is rotated in the forward direction, the command value will be plus and the PWM value will also be plus, so a duty ratio of the PWM signal can be determined. If the duty ratio of the PWM signal is determined, the PWM signal S1 and the forward rotation signal S2 for driving the electric motor 1 will be output from the CPU 26. In response to the PWM signal S1 and the forward rotation signal S2, the first AND circuit 25a of the logic control unit 25 is operated to generate a high-level signal so that the first FET 21 is thereby turned on under the PWM control. Also, in response to the forward rotation signal S2, the fourth FET 24 is turned on. If the first and fourth FETs 21 and 24 are operated or made conductive, a current will flow in a direction from the first FET 21 to fourth FET 24 via the electric motor 1. As a result, the electric motor 1 can be rotated in the forward direction.
If, in the above described state, the driver releases his or her hands from the steering wheel 201, the steering wheel 201 will be rotated in the reverse direction under the action of the caster angle of the front wheels, and the electric motor 1 will be rotated in the reverse direction, so the PWM value will become minus. If the PWM value is minus, it is determined whether the speed of the vehicle at that time is in a low or a high speed region. If the speed of the vehicle is in the low speed region, a duty ratio is then set such that a regenerative current is canceled out, and if the vehicle speed is in the high speed region, the duty ratio will be set to zero. If the duty ratio becomes zero, the first FET 21 will substantially be in its off (non-conductive) state. As a result, a regenerative current flows in a short circuit connecting the third FET 23, the electric motor 1 and the fourth FET 24, so that a regenerative braking force acts on the electric motor 1, thus restricting the returning speed of the steering wheel.
As described above, in the conventional electric power steering control apparatus, the direction of flow of the motor current when the steering wheel is in the returning state is detected and the FETs are controlled according to the result of detection, but in order to detect the motor current direction, there is a need for directly detecting a current flowing through a motor line (i.e., wiring connected to the motor 1) or providing a current detection circuit for detecting a motor current at a location within the motor control H bridge circuit at which the regenerative current can be detected. As a result, there have been problems in that the entire circuitry is complicated and the production cost is increased.
Since the braking force acting against the returning motion of the steering wheel is small, the returning speed of the steering wheel cannot be sufficiently suppressed particularly when the steering wheel is forced to return by an excessive reaction force. Therefore, when the steering wheel is returned at high speeds under the action of an excessive steering wheel return torque, there is the drawback that a burden to the driver is increased.
When the steering wheel is returned by a relatively limited reaction force during low-speed traveling or when the steering angle is large, a braking force develops which restrains the steering wheel return speed so that the driver has to return the steering wheel to the neutral position when the steering wheel is desired to be returned quickly to the, neutral position or it is desired to be returned quickly from the steered state to the straight travel state. As a result, there has been another problem in that the driver is excessively burdened.